1. Field of the Invention
The invention relates to a photochromic coating and to a method for making a photochromic polycarbonate lens using the coating. The coating will exhibit excellent photochromic darkening and fatigue properties.
2. The Prior Art
Injection molded polycarbonate ophthalmic lenses are lightweight and possess excellent mechanical properties. Photochromic dyes are incorporated into certain lenses to enhance their optical properties by automatically lightening or darkening based on the amount and type of light that they receive. Since the photochromic dye molecules can decompose when exposed to the heat of the injection molding process, such dyes need to be introduced on to the lens after the injection molding step. One method is to incorporate the photochromic dye into a coating composition and apply it to a lens surface via dip coating or spin coating. The surface discontinuity of bifocal and trifocal segmented lenses are not well suited to these typical coating processes. The viscous coating material builds-up at the segment which results in an undesirable darker color at the segment when exposed to UV light. A prior art process utilizing a photochromic polyurethane coating is shown in U.S. Pat. Nos. 6,187,444 and 7,258,437. These approaches do not use organic peroxides in the coating compositions, rather they utilize condensation reactions.
Photochromic (PhCh) semi-finished straight-top (SFST) lenses can be made by injection molding polycarbonate (PC) behind a photochromic wafer. The wafer can be a tri-layer of PC/PhCh/PC. VisionEase U.S. Pat. No. 7,036,932 and U.S. Published Patent Application 2007/0122626 describes such a product.
Another method is to overmold a bifocal on top of a lens as described in U.S. Pat. No. 7,258,437. Generally, the overmold casting solution is not photochromic, as mentioned in U.S. Pat. No. 5,531,940, with the lens optionally containing photochromics. As described in U.S. Pat. No. 5,531,940 and JP Patent 2005-199683 the overmolding solution can contain photochromic materials and the lens can be non-photochromic, or a combination of the two. The overmold solution can be cured by either UV, thermal or a combination of each. With these ensembles the desirable high impact nature of the polycarbonate can be compromised because the overmold layer replaces a portion of the total lens thickness.
There is a need to make photochromic polycarbonate bifocal or trifocal segmented spectacle lenses with good photochromic performance.
With the current methods of depositing viscous urethane photochromic coatings as described in U.S. Pat. No. 7,189,456, it is extremely difficult, if not impossible, to produce a polycarbonate photochromic semi-finished segmented (bifocal or trifocal) optical lens. The viscous coating material will build-up at the segment which results in an undesirable darker color at the segment when exposed to UV light.
Another method is where a polycarbonate lens substrate is injection molded. The mold block opens and a few drops of a coating liquid are applied to the front surface of the polycarbonate lens. The mold block then closes. This serves two (2) purposes. First, when the mold block closes, it spreads the coating over the front (CX) surface of the polycarbonate lens to provide a uniform thickness. Secondly, the mold block will provide suitable heat to cure the coating formulation.
There is a need for a photochromic coating formulation and application method which will result in good lens transparency, a high level of photochromic performance, low yellow color and minimal photochromic fatigue.